Recently, there has been a security issue for data stored in an external storage space such as a cloud storage. To protect the data stored in the external storage space, access control or key management is mainly used, which is effective for preventing an external intruder, but fails to fundamentally prevent an operator of the external storage space from maliciously using the stored data. Moreover, if an external user gets a key for accessing the external storage space, the external use may maliciously use the data stored in the external storage space without permission.
To prevent a party other than a person who stores data in the external storage space from maliciously using the data stored in the external storage space, the data may be stored after being encrypted. However, when the data is encrypted and stored, even the person having stored the data in the external storage space may not easily find out the data by using a general data search method, undergoing difficulties in managing the data.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.